JPH0676607B2 - Method for producing ferromagnetic metal powder - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing fine particles of ferromagnetic metal powder as a magnetic material in a magnetic recording medium suitable for high density recording.

Prior art Magnetic powders useful as magnetic tapes and magnetic recording media are γ
-Mainly iron oxide was used, but in recent years, high-density recording media for VTRs and high-class audio have been desired, and powders mainly containing iron oxyhydroxide or iron oxide are subjected to vapor phase contact with a reducing gas. Magnetic powders having a high coercive force mainly composed of metallic iron or cobalt or an alloy of nickel and iron obtained by a reduction reaction have come to be used. The coercive force of metal magnetic fine particles has strong shape anisotropy, and therefore depends on the particle size, acicularity, etc., but coercive force and residual magnetic flux density suitable for tape recording are required.

Magnetic recording media have high output and low noise in a wide recording frequency band regardless of whether they are audio or video, that is, their shape tends to become finer as magnetic powder, and they are compatible with paint resins. It is desired to further improve coating properties, dispersibility, and orientation / filling property of the coating film, and research on improvement of binder resin / various additives and coating dispersion / medium processing technology has been conducted (for example, Goro Akashi “ Advances in Magnetic Tape ", Journal of Japan Applied Magnetics Society, 7 (3), 185 (1983).).

Conventionally, several important techniques are required to obtain a ferromagnetic metal powder mainly composed of iron or iron by heating and reducing it in a reducing atmosphere using a metal compound mainly composed of iron or an iron compound as a starting material. It was

First, in order to prevent sintering caused by heat reduction in a reducing atmosphere and maintain the shape of the raw material, phosphorus, silicon, aluminum, chromium, and a boron compound, etc., are used as starting materials for the iron or iron starting material. It has been proposed to co-precipitate or adhere to a metal hydroxide or oxide mainly composed of and then heat-reduce.

Further, since iron or a metal mainly composed of iron obtained by heating reduction easily oxidizes and burns in the atmosphere as it is, it can be safely handled in the atmosphere and the oxidation of the ferromagnetic metal proceeds. In order to prevent the deterioration of magnetic properties due to heat, it is known to intentionally oxidize the iron or iron-based metal surface obtained by such heat reduction or to deposit and stabilize an inorganic compound. Has been.

Further, it is known to treat the ferromagnetic metal powder with various organic substances or organic metal compounds for the purpose of improving the affinity between the ferromagnetic metal powder and the binder resin for paint.

Problems to be Solved by the Present Invention As described above, the production of a ferromagnetic metal powder sufficiently satisfying various characteristics as a magnetic material for magnetic recording requires an extremely complicated process and is aimed at. It was extremely rare that the addition of the processing technology in expectation of the effect worked independently without affecting other characteristics. As a result, there has been a strong demand for a simple yet inexpensive manufacturing process in manufacturing an excellent ferromagnetic metal powder.

Means for Solving the Problems The inventors of the present invention have sufficiently grasped the conventionally known technology, and as a result of continuing the basic research on the crystal morphology of the oxide layer and the oxide film of the ferromagnetic metal powder, excellent magnetic properties have been obtained. Thus, the present invention has reached the present invention, which has excellent corrosion resistance, has high affinity with the binder resin for coating materials, and has a simple manufacturing process and is inexpensive.

That is, the gist of the present invention is to use iron or a compound containing iron as a main component, for example, an oxyhydroxide or an aluminum compound containing a crystallographically amorphous aluminum compound as a crystallographically amorphous aluminum compound. After the compound is deposited on the surface, it is heated and reduced in hydrogen or a reducing atmosphere mainly containing hydrogen, whereby a metal powder having excellent magnetic properties can be obtained.

Conventionally, it is a known technique to use an aluminum compound in the process of producing a ferromagnetic metal powder, and the aluminum compound is doped in the production of iron oxyhydroxide, which is one of the starting materials. Thing
-30477, 59-19165, and JP-A-56-114833) are known. According to these methods, a raw material having excellent acicularity and particle size distribution can be obtained, and in the subsequent calcination reduction step,
It is said to be effective in preventing interparticle sintering and particle porosity. However, in this case, if the aluminum compound to be doped is increased, the acicularity of iron oxyhydroxide is deteriorated, and magnetite is easily generated, which is not preferable. Also, since aluminum is uniformly dissolved in iron oxyhydroxide, iron spinel (FeAl
No. 204) is produced, and it becomes difficult to reduce, and the saturation magnetization is lowered, which is not preferable.

Further, as a method of surface-depositing an aluminum compound on iron or a metal compound mainly composed of iron, for example, JP-A-56-114833, JP-B-59-19168, and JP-A-59-19163,
57-29523, etc. Of these, JP-A-56-114
No. 833 is a method proposed by the inventors of the present invention, which is a method of simultaneously depositing phosphorus and aluminum, and although an amorphous aluminum compound may be formed, the entrained phosphorus component is the final product. It is recognized that the affinity with the coating resin tends to decrease because the oxidation point of the surface layer of the iron fine particles is formed. On the other hand, Japanese Patent Publications No. 59-19168 and 59-19163
No. 57-29523 and the like are methods in which they are used in combination with a silicon compound, and they are effective to some extent in producing an amorphous silicon compound or an amorphous silicon and aluminum compound. However, if the amount of silicon compound is increased, the hydrophilicity becomes stronger and the affinity with the binder resin decreases, which is not preferable, and if the amount of the aluminum compound to be deposited is increased, it is a crystalline alumina hydrate during the deposition process. So-called gibbsite, vialite, vermite gel, etc. are formed, and the deposition efficiency is reduced. As a method of using the aluminum compound alone, for example, Japanese Patent Publication Sho 56
-28967 etc., but in this case, the essential crystal form of the aluminum compound is not specified or controlled,
As a result, alumina hydrate is often produced and the effect is not manifested in many cases. In addition, in such a method, it is basic to dissolve an aluminum compound in water or an organic solvent and then immerse iron or a metal compound containing iron as a main component. However, there is a problem that the aluminum compound is inevitably segregated during drying and uniform deposition cannot be performed.

Further, as a method for improving the stability of the reduced ferromagnetic metal powder in the air and the affinity with the binder resin, there are, for example, JP-A-58-161709 and JP-A-58-161725, and the present invention provides If it is applied, the effect becomes more remarkable, but there is a drawback that the manufacturing process becomes long.

Furthermore, the affinity between the starting material iron oxyhydroxide or iron oxide and the aluminum compound, especially alumina hydrate, aluminum oxide is extremely high. For example, 10% aluminum oxide is contained in hematite (α-iron oxide). To form a solid solution. In addition, solid solutions are also formed as iron spinels of iron-aluminum composite oxides in magnetite, which is formed during the process of heating and reducing iron or iron-based compounds (Phase Diagrams for Ceramists, Figs 26〜27,2095〜2).
098, The American Ceramic Society, Inc.). Iron spinel is difficult to reduce to metallic iron under normal reducing conditions.
Therefore, when surface-depositing with alumina hydrate, while controlling the crystal morphology and particle size of the alumina hydrate that is the adherend, the solid solution of aluminum oxide to the surface layer of iron oxide or magnetite is controlled. Things are important.

The inventors of the present invention diligently studied a method for controlling the solid solution of aluminum oxide on the surface layer of iron oxide or magnetite, and arrived at the present invention by following the method of controlling the crystal morphology and particle size of alumina hydrate. did.

That is, as a method in an aqueous system, a hydrogel body containing aluminum as a main component containing at least one of phosphorus, silicon and nickel as a secondary component in a weight ratio to aluminum of 1% or more is used in an aqueous system. Adsorbed on the surface layer of metal compound fine particles mainly composed of iron, and then 90 to 130 ° C. at a conventional method to 10 atm or less
Since the uniform deposition by the dense amorphous gel body progresses by heat treatment with, it is possible to suppress the solid solution of aluminum oxide in the surface layer of iron oxide or magnetite in the process of manufacturing the ferromagnetic metal powder. is there. In this case, if the heat treatment condition is less than the above, a dense uniform gel film is not formed. Further, if the heat treatment conditions are exceeded, the crystallization of the gel will substantially progress, and the solid solution of aluminum will be promoted in the subsequent calcination / reduction step, so that the object of the present invention cannot be achieved.

Further, as another non-aqueous method, the metal compound fine particles mainly composed of iron or iron compound is suspended in an organic solvent,
By adding an organoaluminum compound to cause the organoaluminum compound to be adsorbed on the surface layer of the fine particles, and then adding a poor solvent such as acetone or alcohol to make the adsorbed aluminum compound a gel, It is possible to suppress the solid solution of aluminum oxide in the surface layer of iron oxide or magnetite in the process of producing the ferromagnetic metal powder, because the uniform deposition by the crystalline gel body progresses. The latter method is more preferable than the water-based method because it can form an adhered film of only aluminum.

As the aluminum compound used for deposition in the present invention,
Inorganic compounds such as nitrates, sulphates, chlorides, basic acetates or alkali aluminates can be used in the aqueous system, and organic metal compounds such as alkoxide compounds and chelate compounds can be used in the non-aqueous system.

The former surface deposition of the inorganic compound is preferably carried out under the condition that the formation of a hydrate formed by hydrolysis, that is, a crystalline hydrate such as so-called gibbsite, vialite or boehmite gel is suppressed. Further, it is more preferable to prevent the formation of a crystalline hydrate by using in combination with a phosphate such as diammonium hydrogen phosphate or sodium metaphosphate, a silicate such as water glass or an inorganic compound such as nickel or magnesium. You can

When the latter organoaluminum compound is used, when the organoaluminum compound is easily hydrolyzed by water, it is treated in a dry atmosphere, that is, in a water-free state, to give a crystallographically amorphous coating film. You can do it.

The surface-deposited iron or iron-based compound is then calcined at 300 ° C to 800 ° C, if necessary.
-Iron oxide. Calcination at a temperature higher than 800 ° C. is not preferable because the solid solution of aluminum oxide into α-iron oxide becomes remarkable and the interparticle sintering becomes severe.

Next, hydrogen or a reducing gas atmosphere containing hydrogen as a main component is heated and reduced at 300 ° C. to 600 ° C. to produce iron or a ferromagnetic metal powder mainly containing iron. If the temperature is lower than 300 ° C, the reduction is slow and it takes a long time, and if the temperature is higher than 600 ° C, sintering between metal particles becomes severe, which is not preferable.

Reduced iron or iron-based ferromagnetic metal powder will oxidize and burn in the air as it is, so according to the standard method, immerse the metal powder in an organic solvent such as toluene and gradually add oxidizing gas such as air to the metal. The metal surface is oxidatively stabilized by bringing it into contact with powder or by gradually flowing an oxidizing gas such as air in an atmosphere of an inert gas such as nitrogen.

In the present invention, the effect is remarkably exhibited by making the deposited film of the aluminum compound amorphous, but as a suitable deposited amount as a magnetic recording material, Al / Fe = 0.001-0.1 is preferable.

If the deposition amount of the aluminum compound is further increased, the shape of the ferromagnetic metal powder is further improved, but the film becomes thicker, the reduction is slower, and the saturation magnetization is reduced by the weight of aluminum oxide, so that Al / Fe = 0.1 is the limit. Even if the deposited amount of the aluminum compound is small, the effect can be exhibited to some extent, but Al / Fe = 0.001 is the lower limit as the deposited amount suitable as a magnetic recording material.

The ferromagnetic metal powder obtained by the method of the present invention has an amorphous film quality, and as a result, the deposition efficiency is high and uniform, so that the shape retention and magnetic properties are excellent. It is characterized by being excellent in oxidation stability.

Examples Hereinafter, the method of the present invention and its effects will be described in detail with reference to Examples and Comparative Examples.

Example 1 A needle-shaped iron oxyhydroxide was synthesized by hydrolyzing a ferrous sulfate solution with a sodium hydroxide solution by an ordinary method, and then circulating air. Specific surface area (SA) by nitrogen gas adsorption method
Was 72.5 m ² / g, and the ratio of the major axis to the minor axis (L / D) by observation with a transmission electron microscope was 12. The iron oxyhydroxide was diluted and stirred with water, and while maintaining pH 10.0 with an aqueous NaOH solution, sodium hexametaphosphate No. 3, water glass, sodium aluminate, nickel nitrate was used to obtain P / Fe = 0.4 / 100. .Si / Fe = 0.1
/ 100, Al / Fe = 4.0 / 100 and Ni / Fe = 3.0 / 100 were surface-deposited, the pH was adjusted to 8.0 with nitric acid, and then the slurry was boiled at 98 ° C. for 5 hrs, filtered and washed with water. Then, it was dried at 120 ° C. for 18 hours with a box-type hot air dryer, and pulverized with a pulverizer (Nara type free pulverizer) to obtain raw material powder. In order to investigate the crystal morphology of the deposited film, X-ray diffraction measurement was performed on a gel-like product obtained by treating only the deposited reagent containing no iron oxyhydroxide under the same conditions.
As a result, it was found that the quality of the deposited film was amorphous. The surface-deposited iron oxyhydroxide was charged into a fixed-bed type reduction furnace,
Nitrogen gas was circulated at a gas space velocity = 20 Nm ³ -N ₂ /kgr-Fe.Hr, and was calcined at a temperature of 500 ° C for 4 hours, and then subjected to a gas phase catalytic reduction reaction with hydrogen gas (: temperature = 450 ° C, 6 hours, Gas space velocity = 20N
was reduced iron powder by m ³ -N ₂ /kgr-Fe.Hr). Then,
After the fine particles were sufficiently dipped in toluene, the fine particle slurry was transferred onto a enamel vat so that the thickness became about 1 cm, and the toluene was dispersed in the atmosphere. The magnetic powder was collected when the odor of the solvent had disappeared and was used as air-dried metal iron powder.
When observing the shape of the air-dried metal iron powder with a transmission electron microscope, the shape of iron oxyhydroxide as the primary raw material is well inherited on the image, and damage, destruction, and inter-particle sintering are hardly seen. There wasn't. The magnetic properties of the air-dried metal iron powder were measured by a sample vibrating magnetometer (VSM-III) manufactured by Toei Industry Co., Ltd., and the specific surface area was measured by a nitrogen gas adsorption method, Hc = 1500 Oe,
σs = 135 emu / g, R = 0.52, SA = 53.2 m ² / g, and it was found that the magnetic properties were excellent. Further, a corrosion resistance test of the metal powder was conducted. Air-dried iron powder 5.0g temperature 50 ℃, relative humidity 80%
The saturation magnetization (σs) after being kept for 60 hours in a thermo-hygrostat adjusted to 115 was 115 emu / g. It was found that the obtained metal powder has excellent oxidative stability. Further, in order to investigate the affinity with the binder resin for paint, the adsorption amount of vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin S-REC A manufactured by Sekisui Chemical Co., Ltd. was measured. 2.5 g of the air-dried iron powder was dissolved in 1.0 g of S-REC A in a toluene / MEK mixed solution (1: 1) at room temperature for 24 hours, and the adsorption amount was 1.3 mg / m ² , which is a high affinity iron powder. I understand.

Comparative Example 1 The same deposition treatment as in Example 1 was performed, but the boiling treatment was not performed. When the crystal morphology of the deposited film was examined in the same manner as in Example 1, it was found that the crystalline alumina hydrate, vialite (JCPDS Card No. 20-11), was produced.
The characteristics of the metal powder that was calcined and reduced in the same manner as in Example 1 and further subjected to the same gradual oxidation treatment were Hc = 1280 Oe and σs = 122 emu.
/ g, R = 0.47, SA = 50.5m ² / g, and when observing the shape of the air-dried metal iron powder with a transmission electron microscope, partial damage / destruction of the shape of iron oxyhydroxide as a primary raw material, Intergranular sintering was seen. Σs = 103e after corrosion resistance test of the air-dried metal iron powder
The mu / g and the amount of resin adsorbed were as low as 0.8 mg / m ^2, and it was found that the oxidation stability and the resin affinity were inferior to those of Example 1.

Comparative Example 2 Sodium aluminate was Al / Fe = 7.0 / 1 by the method of Example 1.
Only 00 was added to synthesize iron oxyhydroxide. A chemical analysis of the iron oxyhydroxide revealed that it was doped with Al / Fe = 5.6 / 100. SA by nitrogen gas adsorption method is 65.3m ²
/ g, ratio of long axis and short axis (L /
D) was 8. The iron oxyhydroxide was reduced for 10 hours in the same manner as in Example 1 without applying a coating treatment to give an air-dried metal powder. Hc = 880 Oe, σs = 128 emu / g, R = 0.38, SA = 49.
It was 2 m ² / g, and showed unsatisfactory properties even though it was reduced for a long time. When the air-dried metallic iron powder was observed with a transmission electron microscope, remarkable growth of crystallites in particles and inter-particle sintering occurred. It was found to be unsuitable as a magnetic recording material without measuring the oxidation stability and resin adsorption amount of the metal powder.

Example 2 The iron oxyhydroxide of Example 1 was used, diluted and stirred with water, aluminum sulfate was added in an amount of Al / Fe = 3.0 / 100, and urea was added in an amount of 3 times equivalent to the amount of aluminum sulfate, followed by boiling treatment. Was carried out for 15 hours and then filtered and washed with water. As a result of investigating the crystallinity of the deposited film quality in the same manner as in Example 1, it was found to be amorphous. Reduction was carried out in the same manner as in Example 1 to obtain an air-dried metal powder. The air-dried metal powder has Hc = 1470 Oe, σs = 128 emu / g, R = 0.49, SA =
It was 50.6 m ² / g and showed excellent characteristics. When the air-dried metal iron powder was observed with a transmission electron microscope, the shape of the iron oxyhydroxide as the primary raw material was well inherited in the image, and breakage / breakage and inter-particle sintering were hardly seen. Oxidation stability of the metal powder (σs = 113 emu / g) and resin adsorption amount (1.2 mg /
It turns out that m ² ) is also excellent.

Comparative Example 3 Iron oxyhydroxide was diluted with toluene and stirred, and aluminum trisethylacetoacetate was applied only on Al / Fe = 4.0 / 100 and filtered. Then, it was dried under reduced pressure and pulverized. As a result of investigating the crystallinity of the deposited film quality in the same manner as in Example 1, it was found to be amorphous. However, a chemical analysis of the adhered powder revealed Al / Fe = 1.4 / 100, and it was found that the aluminum compound was eluted by filtration. As in Example 4, 37
After reducing at 5 ° C. for 10 hours, the same stabilization treatment as in Example 4 was performed to obtain a metal powder that was not ignitable in the air. The air-dried metal powder has Hc = 1230, σs = 102 emu / g, R = 0.46, SA
= 45.2 m ² / g, showing insufficient characteristics. When the air-dried metal iron powder is observed with a transmission electron microscope, particles having a good inheritance of the shape of the primary raw material iron oxyhydroxide are also observed on the image, but many agglomerates with intense interparticle sintering are observed. From the facts, it was found that uniform deposition was not achieved. It was found to be unsuitable as a magnetic recording material without measuring the oxidation stability and resin adsorption amount of the metal powder.

Example 3 The same iron oxyhydroxide as in Example 1 was dried, diluted with toluene and stirred in a dry air atmosphere, and aluminum isopropoxide was added in an amount of Al / Fe = 4.0 / 100. Further, methanol was added to gel the aluminum compound, and the aluminum compound was deposited, followed by filtration, drying and pulverization. As a result of chemical analysis of the adhered powder, no aluminum compound was eluted by filtration. As a result of investigating the crystallinity of the deposited film quality in the same manner as in Example 1, it was found to be amorphous. The raw material powder was reduced and stabilized under the same conditions as in Example 4 to obtain a metal powder. The metal powder has Hc = 1520, σs = 131 emu / g, R
= 0.50, SA = 54.3 m ² / g, showing excellent characteristics. When observing the metallic iron powder with a transmission electron microscope, the shape of iron oxyhydroxide, which is the primary raw material, is well inherited in the image and damage
Almost no fracture or intergranular sintering was observed. It was found that the oxidation stability (σs = 116 emu / g) and the resin adsorption amount (1.4 mg / m ² ) of the metal powder were excellent.

EFFECTS OF THE INVENTION As is apparent from the above description, in the present invention, iron or a compound mainly containing iron is surface-deposited with a crystallographically amorphous aluminum compound and a compound mainly containing aluminum. After that, by heating and reducing in a reducing gas atmosphere containing hydrogen or hydrogen to produce iron or a ferromagnetic metal powder mainly containing iron, excellent shape retention and magnetic properties and excellent oxidation stability We can provide materials for magnetic recording.

Claims (3)

[Claims] 1. When producing a ferromagnetic metal powder mainly composed of iron or iron by heating and reducing a metal compound mainly composed of iron or an iron compound in a reducing atmosphere, the metal compound is previously crystallized from aluminum. Method of producing a ferromagnetic metal powder, characterized in that the reduction is carried out after a deposition treatment with a substantially amorphous compound or a crystallographically amorphous compound mainly containing aluminum. 2. A phosphorus content of 1% or more by weight of aluminum,
The aluminum-based hydrogel containing silicon or nickel is adsorbed to the surface layer of the metal compound fine particles containing iron or a metal compound containing iron as a main component in an aqueous system, and then the atmospheric pressure to 10 atm or less. 90 to
The method according to claim 1, wherein a dense amorphous gel body is adhered by heating at 130 ° C. 3. Metal compound fine particles containing iron or an iron compound as a main component are suspended in an organic solvent, an organoaluminum compound is added thereto for adsorption treatment on the particle surface, and then a poor solvent is added for adsorption. The method according to claim 1, which accelerates gelation of the aluminum compound.